It is well known that internal combustion engines have revolutionized transportation following their invention during the last decades of the 19th century. While others, including Benz and Gottleib Wilhelm Daimler, invented and developed engines using electric ignition of fuel such as gasoline, Rudolf C. K. Diesel invented and built the engine named for him which employs compression for auto-ignition of the fuel in order to utilize low-cost organic fuels. Equal, if not more important, development of improved spark-ignition engines for use in transportation has preceded hand-in-hand with improvements in gasoline fuel compositions. Modern high performance gasoline engines demand ever more advanced specification of fuel compositions, but cost remains an important consideration.
At the present time most fuels for transportation are derived from natural petroleum. Indeed, petroleum as yet is the world's main source of hydrocarbons used as fuel and petrochemical feedstock. While compositions of natural petroleum or crude oils are significantly varied, all crudes contain sulfur compounds and most contain nitrogen compounds which may also contain oxygen, but oxygen content of most crudes is low. Generally, sulfur concentration in crude is less than about 8 percent, with most crudes having sulfur concentrations in the range from about 0.5 to about 1.5 percent. Nitrogen concentration is usually less than 0.2 percent, but it may be as high as 1.6 percent.
Crude oil seldom is used in the form produced at the well, but is converted in oil refineries into a wide range of fuels and petrochemical feedstocks. Typically fuels for transportation are produced by processing and blending of distilled fractions from the crude to meet the particular end use specifications. Because most of the crudes available today in large quantity are high in sulfur, the distilled fractions must be desulfurized to yield products which meet performance specifications and/or environmental standards. Sulfur containing organic compounds in fuels continue to be a major source of environmental pollution. During combustion they are converted to sulfur oxides which, in turn, give rise to sulfur oxyacids and, also, contribute to particulate emissions.
In the face of ever-tightening sulfur specifications in transportation fuels, sulfur removal from petroleum feedstocks and products will become increasingly important in years to come. While legislation on sulfur in diesel fuel in Europe, Japan and the U.S. has recently lowered the specification to 0.05 percent by weight (max.), indications are that future specifications may go far below the current 0.05 percent by weight level. Legislation on sulfur in gasoline in the U.S. now limits each refinery to an average of 30 parts per million. In and after 2006 the average specification will be replaced by a cap of 80 parts per million maxim.
The fluidized catalytic cracking process is one of the major refining processes which is currently employed in the conversion of petroleum to desirable fuels such as gasoline and diesel fuel. In this process, a high molecular weight hydrocarbon feedstock is converted to lower molecular weight products through contact with hot, finely-divided, solid catalyst particles in a fluidized or dispersed state. Suitable hydrocarbon feedstocks typically boil within the range of 205° C. to about 650° C., and they are usually contacted with the catalyst at temperatures in the range 450° C. to about 650° C. Suitable feedstocks include various mineral oil fractions such as light gas oils, heavy gas oils, wide-cut gas oils, vacuum gas oils, kerosenes, decanted oils, residual fractions, reduced crude oils and cycle oils which are derived from any of these as well as fractions derived from shale oils, tar sands processing, and coal liquefaction. Products from a fluidized catalytic cracking process are typically based on boiling point and include light naphtha (boiling between about 10° C. and about 221° C.), heavy naphtha (boiling between about 10° C. and about 249° C.), kerosene (boiling between about 180° C. and about 300° C.), light cycle oil (boiling between about 221° C. and about 345° C.), and heavy cycle oil (boiling at temperatures higher than about 345° C.).
Not only does the fluidized catalytic cracking process provide a significant part of the gasoline pool in the United States, it also provides a large proportion of the sulfur that appears in this pool. The sulfur in the liquid products from this process is in the form of organic sulfur compounds and is an undesirable impurity which is converted to sulfur oxides when these products are utilized as a fuel. These sulfur oxides are objectionable air pollutants. In addition, they can deactivate many of the catalysts that have been developed for the catalytic converters which are used on automobiles to catalyze the conversion of harmful engine exhaust emissions to gases which are less objectionable. Accordingly, it is desirable to reduce the sulfur content of catalytic cracking products to the lowest possible levels.
The sulfur-containing impurities of straight run gasolines, which are prepared by simple distillation of crude oil, are usually very different from those in cracked gasolines. The former contain mostly mercaptans and sulfides, whereas the latter are rich in thiophene, benzothiophene and derivatives of thiophene and benzothiophene.
Low sulfur products are conventionally obtained from the catalytic cracking process by hydrotreating either the feedstock to the process or the products from the process. Hydrotreating involves treatment of products of the cracking process with hydrogen in the presence of a catalyst and results in the conversion of the sulfur in the sulfur-containing impurities to hydrogen sulfide, which can be separated and converted to elemental sulfur. Unfortunately, this type of processing is typically quite expensive because it requires a source of hydrogen, high pressure process equipment, expensive hydrotreating catalysts, and a sulfur recovery plant for conversion of the resulting hydrogen sulfide to elemental sulfur. In addition, the hydrotreating process can result in an undesired destruction of olefins in the feedstock by converting them to saturated hydrocarbons through hydrogenation. This destruction of olefins by hydrogenation is usually undesirable because it results in the consumption of expensive hydrogen, and also because the olefins are valuable as high octane components of gasoline. As an example, naphtha of a gasoline boiling range from a catalytic cracking process has a relatively high octane number as a result of a large olefin content. Hydrotreating such a material causes a reduction in the olefin content in addition to the desired desulfurization, and the octane number of the hydrotreated product decreases as the degree of desulfurization increases.
Conventional hydrodesulfurization catalysts can be used to remove a major portion of the sulfur from petroleum distillates for the blending of refinery transportation fuels, but they are not efficient for removing sulfur from compounds where the sulfur atom is sterically hindered as in multi-ring aromatic sulfur compounds. This is especially true where the sulfur heteroatom is doubly hindered (e.g., 4,6-dimethyldibenzothiophene). Using conventional hydrodesulfurization catalysts at high temperatures would cause yield loss, faster catalyst coking, and product quality deterioration (e.g., color). Using high pressure requires a large capital outlay. Accordingly, there is a need for an inexpensive process for the effective removal of sulfur-containing impurities from distillate hydrocarbon liquids. There is also a need for such a process which can be used to remove sulfur-containing impurities from distillate hydrocarbon liquids, such as products from a fluidized catalytic cracking process, which are highly olefinic and contain both thiophenic and benzothiophenic compounds as unwanted impurities.
In order to meet stricter specifications in the future, such hindered sulfur compounds will also have to be removed from distillate feedstocks and products. There is a pressing need for economical removal of sulfur from refinery fuels for transportation, especially from components for gasoline.
The art is replete with processes said to remove sulfur from distillate feedstocks and products.
U.S. Pat. No. 2,448,211, in the name of Philip D. Caesar, et al. states that thiophene and its derivatives can be alkylated by reaction with olefinic hydrocarbons at a temperature between about 140° and about 400° C. in the presence of a catalyst such as an activated natural clay or a synthetic adsorbent composite of silica and at least one amphoteric metal oxide. Suitable activated natural clay catalysts include clay catalysts on which zinc chloride or phosphoric acid have been precipitated. Suitable silica-amphoteric metal oxide catalysts include combinations of silica with materials such as alumina, zirconia, ceria, and thoria. U.S. Pat. No. 2,469,823, in the name of Rowland C. Hansford and Philip D. Caesar teaches that boron trifluoride can be used to catalyze the alkylation of thiophene and alkyl thiophenes with alkylating agents such as olefinic hydrocarbons, alkyl halides, alcohols, and mercaptans. In addition, U.S. Pat. No. 2,921,081, in the name of (Zimmerschied et al.) discloses that acidic solid catalysts can be prepared by combining a zirconium compound selected from the group consisting of zirconium dioxide and the halides of zirconium with an acid selected from the group consisting of ortho-phosphoric acid, pyrophosphoric acid, and triphosphoric acid. The Zimmerschied et al. reference also teaches that thiophene can be alkylated with propylene at a temperature of 227° C. in the presence of such a catalyst.
U.S. Pat. No. 2,563,087 in the name of Jerome A. Vesely states that thiophene can be removed from aromatic hydrocarbons by selective alkylation of the thiophene and separation of the resulting thiophene alkylate by distillation. The selective alkylation is carried out by mixing the thiophene-contaminated aromatic hydrocarbon with an alkylating agent and contacting the mixture with an alkylation catalyst at a carefully controlled temperature in the range from about −20° C. to about 85° C. It is disclosed that suitable alkylating agents include olefins, mercaptans, mineral acid esters, and alkoxy compounds such as aliphatic alcohols, ethers and esters of carboxylic acids. It is also disclosed that suitable alkylation catalysts include the following: (1) the Friedel-Crafts metal halides, which are preferably used in anhydrous form; (2) a phosphoric acid, preferably pyrophosphoric acid, or a mixture of such a material with sulfuric acid in which the volume ratio of sulfuric to phosphoric acid is less than about 4:1; and (3) a mixture of a phosphoric acid, such as ortho-phosphoric acid or pyrophosphoric acid, with a siliceous adsorbent, such as kieselguhr or a siliceous clay, which has been calcined to a temperature of from about 400° to about 500° C. to form a silico-phosphoric acid combination which is commonly referred to as a solid phosphoric acid catalyst.
U.S. Pat. No. 4,775,462 in the name of Tamotsu Imai and Jeffery C. Bricker describes a method a non-oxidative method of sweetening a sour hydrocarbon fraction whereby mercaptans are converted to thioethers which are said to be acceptable in fuels. The method involves contacting a mercaptan-containing hydrocarbon fraction with a catalyst consisting of an acidic inorganic oxide, a polymeric sulfonic acid resin, an intercalate compound, a solid acid catalyst, a boron halide dispersed on alumina, or an aluminum halide dispersed on alumina, in the presence of an unsaturated hydrocarbon equal to the molar amount of mercaptans, typically from about 0.01 weight percent to about 20 weight percent. While the product is said to be substantially free of mercaptans, the level of elemental sulfur has not been reduced by this process.
U.S. Pat. No. 5,171,916 in the name of Quany N. Le and Michael S. Sarli describes a process for upgrading a light cycle oil by: (A) alkylating the heteroatom containing aromatics of the cycle oil with an aliphatic hydrocarbon having 14 to 24 carbon atoms and at least one olefinic double bond through the use of a crystalline metallosilicate catalyst; and (B) separating the high boiling alkylation product in the lubricant boiling range from the unconverted light cycle oil by fractional distillation. It also states that the unconverted light cycle oil has a reduced sulfur and nitrogen content, and the high boiling alkylation product is useful as a synthetic alkylated aromatic lubricant base stock.
U.S. Pat. No. 5,599,441 in the name of Nick A. Collins and Jeffrey C. Trewella describes a process for removing thiophenic sulfur compounds from a cracked naphtha by: (A) contacting the naphtha with an acid catalyst to alkylate the thiophenic compounds using the olefins present in the naphtha as an alkylating agent; (B) removing an effluent stream from the alkylation zone; and (C) separating the alkylated thiophenic compounds from the alkylation zone effluent stream by fractional distillation. It also states that additional olefins can be added to the cracked naphtha to provide additional alkylating agent for the process.
More recently, U.S. Pat. No. 6,024,865 in the name of Bruce D. Alexander, George A. Huff, Vivek R. Pradhan, William J. Reagan and Roger H. Cayton disclosed a product of reduced sulfur content which is produced from a feedstock which is comprised of a mixture of hydrocarbons and includes sulfur-containing aromatic compounds as unwanted impurities. The process involves separating the feedstock by fractional distillation into a lower boiling fraction which contains the more volatile sulfur-containing aromatic impurities and at least one higher boiling fraction which contains the less volatile sulfur-containing aromatic impurities. Each fraction is then separately subjected to reaction conditions which are effective to convert at least a portion of its content of sulfur-containing aromatic impurities to higher boiling sulfur-containing products by alkylation with an alkylating agent in the presence of an acidic catalyst. The higher boiling sulfur-containing products are removed by fractional distillation. It is also stated that alkylation can be achieved in stages with the proviso that the conditions of alkylation are less severe in the initial alkylation stage than in a secondary stage, e.g., through the use of a lower temperature in the first stage as opposed to a higher temperature in a secondary stage.
U.S. Pat. No. 6,059,962 in the name of Bruce D. Alexander, George A. Huff, Vivek R. Pradhan, William J. Reagan and Roger H. Clayton disclosed a product of reduced sulfur content is produced in a multiple stage process from a feedstock which is comprised of a mixture of hydrocarbons and includes sulfur-containing aromatic compounds as unwanted impurities. The first stage involves: (1) subjecting the feedstock to alkylation conditions which are effective to convert a portion of the impurities to higher boiling sulfur-containing products, and (2) separating the resulting products by fractional distillation into a lower boiling fraction and a higher boiling fraction. The lower boiling fraction is comprised of hydrocarbons and is of reduced sulfur content relative to the feedstock. The higher boiling fraction is comprised of hydrocarbons and contains unconverted sulfur-containing aromatic impurities and also the higher boiling sulfur-containing products. Each subsequent stage involves: (1) subjecting the higher boiling fraction from the preceding stage to alkylation conditions which are effective to convert at least a portion of its content of sulfur-containing aromatic compounds to higher boiling sulfur-containing products, and (2) separating the resulting products by fractional distillation into a lower boiling hydrocarbon fraction and a higher boiling fraction which contains higher boiling sulfur-containing alkylation products. The total hydrocarbon product of reduced sulfur content from the process is comprised of the lower boiling fractions from various stages. Again it is stated that alkylation can be achieved in stages with the proviso that the conditions of alkylation are less severe in the initial alkylation stage than in a secondary stage, e.g., through the use of a lower temperature in the first stage a opposed to a higher temperature in a secondary stage.
The need for removing certain nitrogen compounds upstream of various processes has also been recognized in the art.
For instance, U.S. Pat. No. 6,602,405B2 (Pradhan et al.) discloses process for producing products having a reduced sulfur content wherein basic nitrogen containing impurities are removed from the feed stock prior to passing the feedstock to an olefin-modification reaction zone using a solid phosphoric acid catalyst or an acidic polymeric resin catalyst.
U.S. Pat. No. 6,599,417B2 (Pradhan et al.) similarly teaches the removal of basic nitrogen containing impurities from a feedstock prior to passing the feedstock to an olefin modification reaction zone.
U.S. Pat. No. 6,736,660B2 (Pradhan et al.) also teaches the removal of nitrogen-containing organic compounds upstream of an acidic catalyst process.
While the prior art is cognizant of the need to remove nitrogen-containing molecules upstream of an acid catalyst based process the prior art has not recognized that the impact of non-basic nitrogen-containing organic compounds is even more severe as a catalyst poison versus basic or neutral nitrogen compounds.
Typical methods disclosed in art for removing nitrogen-containing molecules such as acid wash steps or acidic guard bed techniques will not work to remove these highly deleterious non-basic nitrogen compounds.
There is, therefore, a present need for processes to prepare products of reduced sulfur content from a feedstock wherein the feedstock is comprised of limited amounts of sulfur-containing and non-basic nitrogen-containing organic compounds where the deleterious non-basic nitrogen compounds can be readily removed before they can act as a catalyst poison.
This invention is directed to overcoming the problems set forth above in order to provide components for refinery blending of transportation fuels friendly to the environment.